More precisely, the invention relates to an exhaust casing for a turbomachine, the casing comprising:                a hub centered on an axis and carrying, on each of its upstream and downstream faces, an annular endplate that is coaxial with the hub;        an outer shroud coaxial with the hub; and        a plurality of arms interconnecting said hub and said shroud, each arm presenting opposite first and second side faces in which the hub and the outer shroud are spaced apart radially by a length written L;        
in which, in a radial section plane, the first side face of each arm forms an acute angle with the tangent to the outer periphery of each endplate, said angle lying in the range 60° to 85°; and
in which the first side face of each arm is connected to each endplate via a connection portion.
In the present application, “upstream” and “downstream” are defined relative to the normal flow direction of fluid flowing through the turbomachine (from upstream to downstream). Furthermore, the axial direction corresponds to the direction of the axis of rotation A of the turbomachine rotor, and a radial direction is a direction perpendicular to the axis A. Similarly, an axial plane is a plane that includes the axis of rotation A, and a radial plane is a plane perpendicular to the axis A. Finally, unless specified to the contrary, adjectives such as “inner” and “outer” are used with reference to a radial direction such that an inner (i.e. radially inner) portion of an element is closer to the axis A than is the outer (i.e. radially outer) portion of the same element.
Accompanying FIGS. 1 to 3 show an example of an exhaust casing 12 of the above-specified type.
FIG. 1 is a view of a two-spool bypass turbojet 1 in axial section, i.e. in section on a plane containing the axis of rotation A of the rotor of the turbojet. From upstream to downstream, the turbojet 1 comprises: a fan 2; a low-pressure compressor 4; a high-pressure compressor 6; a high-pressure turbine 8; a low-pressure turbine 10; and said exhaust casing 12.
The casing 12 is thus situated downstream from the low-pressure turbine 10. The casing 12 contributes to defining the primary flow section of the fluid passing through the turbojet. Furthermore, the casing 12 supports the rotor of the turbojet and ensures that the rotor and the stator are coaxial. Finally, the oil that lubricates the bearings of the turbojet is fed and removed via the casing 12.
With reference to FIGS. 2 and 3, the casing 12 comprises:                a hub 14 centered on the axis A and carrying on its upstream and downstream faces respective upstream and downstream annular endplates 16 and 18, coaxial with the hub 14;        an outer shroud 20 coaxial with the hub 14; and        a plurality of arms 22 connecting the hub 14 to the shroud 20 (there being sixteen arms in the example shown).        
The upstream and downstream annular endplates 16 and 18 are contained in substantially radial planes and they project outwards towards the shroud 20.
Each arm 22 presents first and second opposite side faces 22a and 22b. In a radial plane, such as the plane of FIG. 3, the first side face 22a of each arm 22 forms an acute angle “a” with the tangent to the outer periphery of each endplate 16, 18, the angle “a” lying in the range 60° to 85°. Furthermore, the first side face 22a of each arm 22 is connected to each endplate 16, 18 via a connection portion 24.
The lifetime of presently known exhaust casings is found to be insufficient, and an object of the invention is to increase said lifetime.
In their research leading to the present invention, the inventors found that when the turbojet is in operation, the exhaust casing is subjected to a temperature gradient between the outer shroud and the hub, which temperature gradient gives rise to stresses in the connection portion; and since the level of stress reached in this connection portion is high, the lifetime of the casing suffers therefrom.
Until now, in known exhaust casings, the profile of the connection portion in a radial section plane has been defined by a circular arc having a radius of 3 millimeters (mm).